Any diesel that cranks but does not fire and run has only two levels of possible issue as I have discovered in 60+ years of taking care of diesels of all sizes.
Most likely is a fuel issue and the easiest is that a filter was changed and the system no bleed as necessary for the injectors to get to opening pressure. This is an easy fix if you know how to do it. There are other fuel system possibilities that may require part$.
Less likely is a fault in the engine like a hung valve (real bad in any diesel) and as soon as the engine has to come apart, you are into money.
If you do buy it, take it to a boat shop. Those people deal with miniature engines (from my point of view) all the time and maybe even that same one.
Matt